1. Field of the Invention
The present invention is directed to cryogenic refrigeration cycles such as the Stirling and Gifford-McMahon cycles which use a cryogenic heat regenerator. It is more specifically directed to a new class of materials that can be used for storing and discharging heat in a cryogenic regenerator at temperatures below 12 K.
2. Description of the Prior Art
The ideal cryogenic regenerator is a flow-through device which can quickly absorb heat energy from a flowing refrigerant during a first refrigeration cycle, efficiently store the absorbed heat for a length of time, and quickly return the absorbed heat to the refrigerant during another refrigeration cycle. Such an ideal cryogenic refrigerator should be capable of transferring heat energy to and from the refrigerant as quickly as possible and should simultaneously be capable of storing within its volume a substantial portion of the heat energy carried by a corresponding volume of the refrigerant. Numerous attempts have been made to obtain such characteristics at temperatures below 12 K. but the attempts have been at best, only partially successful.
The problems of previously attempted regenerators will be described with reference to FIG. 1 in which there is shown a flow diagram of a known Gifford-McMahon refrigeration system 5 which includes a commonly used cryogenic regenerator 10. The cryogenic regenerator 10 comprises a flow-through chamber 12 filled with a matrix of tiny lead balls 14. The balls 14 are used for absorbing and storing heat from a volume of a flowing-through refrigerant 40. Voids between the balls define a flow-through gap which is schematically indicated at 15. The surface area of the balls 14 determines the heat exchange area of the gap 15. It will be understood that the heat capacity of this regenerator 10 is a function of numerous factors including what percent volume of the chamber 12 is occupied by the lead balls 14 and what mass each ball has. There is a general need within the cryogenic arts to maximize the heat capacity of such a regenerator 10 so that an energy efficient refrigeration system can be constructed.
The chamber 12 is elongated to permit the establishment of a temperature gradient T.sub.upper -T.sub.lower between respective upper and lower regenerator ports, 16 and 18, disposed at opposed ends of the chamber 12. The upper and lower temperatures, T.sub.upper and T.sub.lower, can vary from system to system, but by in large, the lower temperature T.sub.lower has until now been generally limited, for practical applications, to temperatures above 12 K. The importance of this temperature floor will be explained later. It is sufficient for now to state that there has been a long felt desire within the cryogenic field to find an efficient way to lower this temperature floor and that numerous attempts have been made by others to do so.
A displacement chamber 20 having opposed upper and lower displacement ports, 26 and 28, is connected to the corresponding upper and lower ports of the regenerator 10 to form a heat regenerative loop 21 as shown in FIG. 1. The loop 21 includes a lower heat exchanger 23 disposed between the lower regenerator port 18 and the lower displacement port 28. A displacer piston 24 divides the displacement chamber 20 into upper and lower displacement areas, 25 and 27. The displacer piston 24 is driven by a motor 22 which reciprocates the piston 24 upwardly and downwardly to thereby change the volumes of the upper and lower displacement areas, 25 and 27.
A compressor 30, incorporating a reciprocal piston 31 that is designed to compress a cryogenic refrigerant 40 such as helium, is coupled to the regenerative loop 21 through a pair of conduits and a series of electrically controlled valves 32A, 32B, 32C and 32D. An upper heat exchanger 33 is interposed in a first of the conduits between the compressor 30 and the regenerative loop 21.
The operation of the illustrated refrigeration system 5 will now be explained. Valves 32A-32D are initially closed. Input power W.sub.in is applied to the compressor 30 to compress the refrigerant 40. Compression causes the temperature of the refrigerant to rise above an output temperature level T.sub.out existing at the upper heat exchanger 33. Valves 32A and 32B are opened. The heated refrigerant 40 flows through the upper heat exchanger 33 where it releases a certain amount of output heat energy Q.sub.out at the output temperature level T.sub.out. At the same time, the displacer motor 22 is actuated to move the displacer piston 24 downwardly so that the refrigerant 40, which has just been cooled to the output temperature level T.sub.out in the upper heat exchanger 33, enters the upper displacement area 25.
Valves 32A-32D are closed. The displacer piston 24 is then moved upwardly to force the refrigerant 40 in the upper displacement area 25 to flow clockwise through the illustrated regenerative loop 21. The refrigerant 40 is forced through the regenerator 10 and the lower heat exchanger 23 into the lower displacement area 27 of the displacement chamber. The lead balls 14 of the regenerator are precooled to temperatures in a range T.sub.upper -T.sub.lower below T.sub.out so that the lead balls can absorb a first amount of heat energy Q.sub.1 from the refrigerant as it flows through in the clockwise direction. By the time it exits the lower regenerator port 18, the refrigerant 40 should be cooled to approximately the lower temperature T.sub.lower of the regenerator 10. The cooled refrigerant is temporarily held in the lower displacment area 27.
Valves 32C and 32D are next opened and the compressor piston 31 is moved upwardly to reduce pressure in the loop 21. As a result of this pressure reduction, the temperature of the refrigerant in the lower displacement area 27 drops a second time, to a new temperature below the initial lower temperature T.sub.lower of the regenerator. The displacement piston 24 is reciprocated downwardly and the twice cooled refrigerant within the lower displacement area 27 is then forced back through the lower heat exchanger 23 and upwardly through the regenerator 10 into the compressor 30. As the refrigerant 40 passes upwardly through each of the lower heat exchanger 23 and regenerator 10 it absorbs respective amounts of heat energy Q.sub.in and Q.sub.2 from each these loop components. The corresponding temperatures of the lower heat exchanger, T.sub.in, and the lower temperature, T.sub.lower, of the regenerator are reduced by this heat absorbing step.
In a subsequent cycle, the reduced lower temperature T.sub.lower of the regenerator 10 is used to further cool the refrigerant when the refrigerant 40 again moves from the upper displacement area 25 in a clockwise direction through the loop 21 into the lower displacement area 27. The refrigerant 40 becomes cooler and cooler with each successive cycle until physical limits of heat exchange between the refrigerant flowing through the regenerator and the heat absorbing/discharging material (lead balls 14) of the regenerator 10 are reached.
For prior art regenerators which use a lead ball matrix 14 this temperature limit occurs at approximately 12 degrees Kelvin. Some devices have been built which reduce the temperature floor to 10 K. or less but only with a substantial loss of operating effeciency (W.sub.in versus Q.sub.in). The temperature floor is believed to result primarily from the fact that the volumetric specific heat of lead (Pb) drops below that of a pressurized refrigerant composed of free flowing helium at approximately 12 K. (FIG. 2). Numerous attempts have been made to use materials having volumetric specific heats larger than that of lead but these attempts have created other problems. The higher heat capacity obtained from previously tested materials has been offset by heat conduction characteristics inferior to those of lead (Pb). The previously tested materials include thallous and cerium-halide materials, gadolinium compounds, neodymium and europium-sulfide materials, microspherical silica gel, ultra fine lead particles and even ordinary charcoal.
One reason why there is a desire to develop a regenerator which can operate efficiently below 12 K. will now be explained. In various cryogenic applications, such as the cooling of infrared detectors aboard an orbital platform (space satellite), cooling to temperatures very close to absolute zero (i.e. 4 K.) is required for the infrared detectors to operate at a desired sensitivity level. Previously, a secondary refrigeration cycle employing a gas expansion device known as a Joule-Thomson valve was used to reach temperature levels below 12 K. Known Stirling-type refrigeration systems were incapable of reaching such temperatures in an efficient manner.
The Joule-Thomson valve relies on a very tiny orifice which is used for the expansion cooling of helium to temperatures as low as 2 K. The orifice is prone to clogging by contaminants such as trace particles of lubricating oils that may enter the refrigerant carrying conduits of a J-T system. Orifice clogging is believed to be a primary factor responsible for a known propensity of J-T refrigeration systems to malfunction. A solution to this clogging problem has not yet been found.
Helium J-T refrigerators have a mean time between maintenance (MTBM) of roughly 2,000 hours. This MTBM makes the J-T systems impractical for cooling remotely located sensing devices such as those placed aboard an orbiting satellite. In contrast, the Gifford-McMahon and Stirling type refrigeration systems, which do not rely on a tiny orifice, can be built to have MTBM's on the order of approximately 20,000 hours or better. The present invention makes possible a refrigeration system that can operate efficiently and attain both a relatively high MTBM and a temperature below 12 K. Heretofore, the efficient range of operation of such higher MTBM systems was generally limited to temperatures above 12 K.